Sodium sulfate (Na2SO4) is a commodity product widely used in the detergent and paper making industries.
Large volumes of waste waters containing Na2SO4 are generated each year in the manufacture of precipitated silica. So far, in view of its abundant availability and reduced cost, the recovery of Na2SO4 from these waste waters has not been economically attractive.
Precipitated silica is a synthetic, finely divided, white, amorphous form of silicon dioxide. Precipitated silica is typically obtained via the reaction of an alkaline silicate solution with a mineral acid. In a typical process, sulfuric acid and sodium silicate solutions are reacted under agitation according to the following reaction scheme:Na2O.3.3SiO2+H2SO4→3.3SiO2+Na2SO4+H2O
The resulting precipitate is filtered, washed and dried in the manufacturing process.
The precipitated silica production process generates large volumes of wastewater containing large amounts of dissolved sodium sulfate. Although sodium sulfate is environmentally safe, the possibility to recover it from precipitated silica wastewaters in an economically viable fashion would represent an advantageous improvement of the overall precipitated silica manufacturing process.
Large amounts of heat are also required to dry the wet precipitated silica at the end of the manufacturing process. Drying may be carried using any known technique, such as spray drying, fluid bed drying, rotary drying and the like, wherein a suspension containing the precipitated silica particles is dried by means of hot dry air. At the end of the drying process large volumes of hot gas, mainly water vapour, are discharged from the drying apparatus and typically wasted.
It has now been found that it is possible to use the residual heat of the drying step in a precipitated silica manufacturing process in a membrane distillation process which allows recovering Na2SO4 from a water stream.
The process is particularly advantageous when the water stream containing Na2SO4 is produced in the precipitated silica manufacturing process. The combination of the heat discharged from the drying step with the membrane distillation process allows an economically advantageous route for the recovery of Na2SO4 from water generated in the precipitated silica manufacturing process.
It has been found that the temperature differential required for a membrane distillation process for the recovery of Na2SO4 from water, in particular from water produced in a precipitated silica manufacturing process, can be advantageously obtained by using the residual heat contained in the flow of hot gas discharged from the drying step of the precipitated silica manufacturing process itself.